This invention relates to microwave radio repeaters for telecommunications, and more specifically to an inexpensive nonfrequency-converting radio repeater system having very low power requirements.
Microwave radio repeater systems are used to amplify and redirect the RF carriers where the distance between radio terminals is extreme and where geographical and manmade obstacles block the line-of-sight transmission path. Generally, microwave repeaters may be classified as either active or passive. The more common type of passive repeater is the flat "billboard" type metal reflector which acts as a simple microwave mirror. The passive repeater is very useful, where the topography permits, and where it is suitable in other respects for the particular system application. The principal reasons for using a passive repeater are that it provides a fixed signal gain, it is very reliable, and it requires virtually no maintenance. Thus, it can be used on short microwave hops which are inaccessible or too expensive for the typical active repeater. However, for a variety of reasons (cost being the biggest factor), certain applications of passive repeaters are even more impractical than active repeaters both to install and operate. In frequency-congested areas, billboard reflectors can cause interference with adjacent systems since a billboard reflects all electromagnetic energy independent of the source. Furthermore, passive repeaters cannot be used effectively, for reasons of low RF gain, in the 2 GHz band or below or where the included angle exceeds 140.degree..
Currently, the active repeaters in most radio relay systems are either of the demodulating-remodulating "baseband" type or the IF or RF heterodyne type. In each case, the individual information-carrying channels are amplified and otherwise processed at frequencies usually lower than the microwave carrier frequency. For such active repeaters, the overall costs can be prohibitive for inaccessible repeater locations. Aside from the initial radio equipment cost, access roads for routine maintenance must be built and maintained, utility power lines or local power generators must be installed, and repeater housings also constructed. The invention disclosed herein offers an economical and reliable alternative to the traditional active microwave repeater for applications in remote locations where passive repeaters might otherwise be used.
An inexpensive microwave radio repeater providing 25 to 50 dB of gain (not including antenna gain) and capable of being powered from solar cells and/or primary cells could offer a very practical alternative to the passive repeater in the application described above. Since solar cells are low-voltage devices and are capable of supplying only very little power, a radio repeater powered from such cells must be extremely uncomplicated and have very low power requirements, A nonfrequency-converting radio repeater using a single stage of amplification can fulfill all these requirements.
Nonfrequency-converting duplex (bidirectional) repeaters, similar in configuration to the type used herein, have been used commercially for years in the communication industry on submarine cable systems and on CATV cable systems. See "System Considerations in the Design of a Two-Way Transmission System", by H. B. Marron and A. W. Barnhart in the 19th Annual NCTA Convention Transcript 1970, pp. 547- 571. For reasons of poor amplifier response and high echo distortion, such repeaters have been limited to cable TV systems and low frequency communication systems. A recent discussion of a single channel, nonfrequency-converting duplex repeater in the VHF/UHF band (225 MHz to 468 MHz) may be found in "Short-Hop Radio-Relay Systems Using Tunnel-Diode Repeaters" by D. L. Hedderly et al., Proceedings IEE, vol. 114, April 1967, pp. 435 - 442. This experimental system for use on electrical transmission high-voltage towers used tunnel diode amplifiers and yagi aerials with curtain reflectors, both of which are very impractical for high-quality reliable microwave radio telecommunication operation of the type discussed herein.
A multichannel nonfrequency-converting repeater has never been used on a commercial basis for microwave radio communications in the common carrier bands above 1.7 GHz. The reason for this is that the many technical problems involved have been thought to be insurmountable. A high-level echo having a long time delay resulting from transmitting and receiving on the same carrier frequency can cause severe echo distortion in the radio baseband. And, this is true independent of the form of carrier modulation used. The invention herein minimizes the echo distortion to acceptable levels, making the nonfrequency-converting repeater a practical concept. Also, by using a reliable repeater design having absolute minimum power requirements, the entire repeater can be used in remote locations without the necessity of locally generated utility-supplied AC power.